The invention primarily relates to a diaphragm-type carburetor for supplying fuel to general-purpose engines and, more particularly, relates to a diaphragm-type carburetor comprising a butterfly-type throttle valve and a single fuel nozzle that allows fuel measured in accordance with the opening or closing of the throttle valve to be sent from the fuel nozzle.
Two- and four-cycle general-purpose engines are small in size, and small diaphragm-type carburetors are often used to supply fuel thereto. Examples of commonly known diaphragm-type carburetors are the fixed venturi model discussed in Japanese Kokai S55-69748, which comprises a butterfly-type throttle valve and two fuel systems, a low-speed system and a main system; the variable venturi model presented in Japanese examined utility model application No. S49-17682, which comprises a single fuel system capable of supplying a variable amount of fuel by means of a cylindrical sliding throttle valve and a metering pin attached to the sliding throttle; and the carburetor described in Japanese Kokai S58-101253, which comprises a single fuel system capable of supplying a variable amount of fuel by means of a cylindrical rotary throttle valve and a metering pin attached to the rotary throttle valve.
The control of the fuel supply by the metering pin in response to axial movement of the sliding or rotary throttle valve in the single fuel system models is beneficial in that it requires no special consideration for fuel-related connections and, unlike the models with two fuel systems, includes a simple pathway structure. In addition, the cross sectional area of the sliding or rotary throttle valve, when fully open, is identical to that of the air intake pathway, thus beneficially allowing such models to more easily supply the required volume of air at times of high output than the fixed venturi model.
The sliding valve linearly reciprocates along a length nearly identical to the diameter of the air intake pathway. As a result, a spacing of a size at least equivalent to the stroke of the sliding throttle valve must be provided between a constant fuel chamber, which contains a constant amount of fuel by means of a diaphragm, and the opening of the fuel nozzle to the air intake pathway in order to accommodate a metering pin that operates integrally with the sliding throttle valve. For this reason, the air intake pathway cannot be made sufficiently small. As far as the rotary throttle valve, it moves slightly in the central axial direction as it rotates so that a metering pin that moves integrally with the rotating throttle valve can control the amount of fuel supplied. Because minute movements of the metering pin control the required fuel amounts for all operating levels of the engine, the dimensional and positional relationships between the fuel nozzle and the metering pin have to be set with a high degree of accuracy, which poses design and manufacturing problems.
The present invention was created in order to solve the above problems of the fixed venturi, sliding throttle valve, and rotary throttle diaphragm-type carburetorsxe2x80x94including those problems related to a transition to high output, fuel-related connections, miniaturization of the carburetor, and design and construction simplicity. A primary object of the present invention is to provide a diaphragm-type carburetor that enables high output and miniaturization of the carburetor as a whole, yet poses no particular design or manufacturing problems.
In order to solve the above problems, the present invention provides a diaphragm-type carburetor comprising an air intake pathway that penetrates a body and is formed with a nearly uniform diameter along its entire length, a constant fuel chamber that is provided along one face of the body and contains a constant amount of fuel by means of a diaphragm, a butterfly-type throttle valve that opens and closes the air intake pathway, a fuel nozzle that is disposed on the downstream side of the throttle valve and supplies fuel introduced from the constant fuel chamber to the air intake pathway, a metering pin having a tip thereof inserted into the fuel nozzle, a cam member centered on a valve stem of the throttle valve and having an arc-shaped cam face, and an actuating member that makes constant contact with the cam face and reciprocates linearly. The metering pin, which is held by the actuating member, reciprocates linearly following the opening and closing of the throttle valve, and controls the amount of fuel supplied from the fuel nozzle to the air intake pathway.
Because the air intake pathway lacks a venturi and has a nearly uniform diameter along its entire length, it can easily provide the airflow rate required during high output. Additionally, because the throttle valve is a butterfly-type throttle valve, the valve stem length is shorter than the sliding and rotary models. This allows for miniaturization of the carburetor as a whole. Moreover, the fuel nozzle is positioned on the downstream side of the throttle valve and the fuel supply amount is controlled by the metering pin, which follows the throttle via a cam mechanism. Therefore, the required fuel rate can be controlled over an entire operating range of the engine with a single fuel system. In this case, the stroke of the metering pin may be set as desired with the cam irrespective of the throttle valve. As a result, the function of appropriately controlling the amount of fuel supplied over the entire operation range of the engine can be easily provided.
In the above embodiment of the invention, the actuating member has a contact portion that makes contact with the cam face and a retaining member for retaining the metering pin, and is supported on the body by a rotation locking means. The force of a spring acts to place the contact portion in contact with the cam face. The retaining member, which has the shape of an open-ended tube, is positioned in a region outside of the cam member. The retaining member retains the metering pin so that the insertion depth thereof into the fuel nozzle can be adjusted by an adjustment screw screwed into the interior thereof. The retaining member configuration is preferred for smooth and accurate conversion of the opening and closing motion of the throttle valve into linear reciprocating motion of the metering pin and that also for appropriate adjustment of the insertion depth of the metering pin into the fuel nozzle after assembly.
In the above embodiment of the present invention, the throttle leverxe2x80x94which is attached to the valve stem so that movement associated with acceleration control is transmitted to and opens or closes the throttle valvexe2x80x94preferably acts as a cam member as well in order to reduce the number of parts.